Problem Biasing

  • Thread starter Thread starter Marco 2073
  • Start date Start date
M

Marco 2073

New member
Just 3 months ago I got a wonderfull new SmallBox from MusikProduktiv....
Testing the values of the stock power tubes (ARS - JJ Tesla) I read values not matched: one tube 27mA and the other 35mA.
Once get new tubes TAD EL34B-STR "premium quality" I tried to re-bias the amp. Unfortunately I cannot set it less than 40mA due to the limit of the bias Trim of the amp.
According to the seller the problem should have been the "plate current" of the new tubes (PC=50) which was too high.
Thus the seller sent me new tubes with lower plate current (PC=35) but I find out the same problem. This time I cannot set the bias lower than 39ma.

Before re-contact the seller I wanted to ask you if the problem could be on the bias trim or do you tihnk are the tubes? With the stock ARS I can set the trim starting from 19mA.....

Thank you for your help.
 
A tech can just change a resister in the amp and get you in the range. The resister feeds the bias pot it's a 47k just up it to a 68k.
 
So the resister could be different for each Small box amp?
 
No we just use tubes in a certain range that work.. Nothing wrong with changing the resistor though. Might be changing to a 50k bias pot soon so there is more range.
 
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